1. Field of the Invention
This invention relates to an information reproduction apparatus, in particular this invention relates to an information reproduction apparatus for reproducing an FM signal by FM demodulating with detection of an inversion phenomenon developed in FM demodulation processing and with compensation of the demodulated FM signal distorted due to the inversion phenomenon, the FM signal being supplied from a VTR such as S-VHS ("S-VHS" is a trademark for VTR) and the like or from a transmission system for transmitting a video signal FM-modulated.
2. Description of the Prior Art
All information reproduction apparatus is known which comprises an FM demodulator and a deemphasis circuit. In a video tape recorder (VTR) for general home use as a typical information reproducing apparatus, an FM modulator is provided in a luminance signal processing circuit for recording. On the other hand, an FM demodulator is used in the luminance signal processing circuit for reproduction. There is a problem of an inversion phenomenon (problem) which should be taken care for the FM demodulation of the VTR. The higher the frequency of the video signal, or the more the recording current increases, or the higher the frequency of a carrier (carrier), the more unique characteristics it shows, which characteristics are not found in the general signal transmission lines. Thus, this causes an amplitude-frequency characteristic to deteriorate or the inversion problem, so that distortion is generated.
FIGS. 3A to 3E show waveforms for explaining the inversion phenomenon in the prior art. The inversion phenomenon is as follows:
When there is a glitter portion on the reproduced image from the VTR, an edge of such portion on the reproduced image seems broken.
If there is unbalance between upper and lower side band waves of the reproduced FM signal, and when luminance represented by the video signal changes from black to white, the waveform of the FM signal distorted at a steep rising portion thereof such that a half cycle of a portion of the waveform does not cross the zero level but swings as shown in FIG. 3C. If this distorted signal is applied to a limiter as it is, the FM signal is equivalently demodulated such that in its waveform, the luminance does not rise toward the white level but falls to the black level because the zero-cross point having information is lacked. Thus, this is referred to as "inversion phenomenon" or "inversion problem". In particular, if the video emphasis is effected, this phenomenon occurs very often because a spike is developed after a rising portion of a waveform of a video signal. FIGS. 3A to 3E show this phenomenon. FIG. 3A shows an FM signal representative of an edge portion where a spike is developed by the video emphasis. FIG. 3B shows a waveform of an FM signal representative of that edge portion. FIG. 3C shows a waveform of a reproduced FM signal representative of the edge portion. FIG. 3E shows a waveform of an output of an FM demodulator representative of the edge portion where the inversion phenomenon occurs.
The inversion phenomenon results in limitation of a frequency shift amount .DELTA.f and a video emphasis amount. Thus, various countermeasures against the inversion phenomenon have been tried in the recent home use VTRs in order to prevent the inversion phenomenon even if the video emphasis amount is increased. For example, the inversion phenomenon is prevented as follows:
A reproduced FM signal is applied to a circuit for suppressing its low frequency components and for emphasizing its high frequency components because this makes a level of the carrier smaller than a level of the Lower sideband to prevent the zero-cross error shown in FIG. 3C. However, an SN ratio of the demodulated signal will decrease because lower sideband components having a higher SN ratio are suppressed. Then, an anti-inversion circuit has been adopted which operates the low frequency suppression circuit only in the condition that the inversion may occur, that is, it effects the low frequency suppression circuit when decrease in the level of the carrier is detected.
However, such circuits do not sufficiently prevent or compensate the inversion phenomenon. This is because these circuits mentioned above detect only decrease in the level of the carrier to prevent the inversion problem, so that these circuits also detect decrease in the level of the FM signal due to dropout, etc. Moreover, it is uncertain that the inversion always occurs when the level of the FM signal decreases. Further, there is also a problem that the compensation for the inversion problem was not sufficient. In particular, there is the problem of the inversion phenomenon in the compatibility-reproducing or in the long time reproducing mode because the picture quality decreases considerably.